Process of recovering high-grade phosphoric acid.



material and dust FRAN K S. WASHBUBN, OF NASHVILLE, TENNESSEE.

PROCESS OF RECOVERING- HIGH-GRADE PHOSPHORIC ACID.

No Drawing.

To all whom it may concern:

Be it known that I, FRANK S. WAsHBURN, a citizen of the United States,residing at Nashville, in the county of Davidson and State of Tennessee,have invented certain new and useful Improvements in Processes ofRecovering High-Grade Phosphoric Acid; and I do hereby declarethefollowing to be a full, clear, and exact description of the invention,such as will enable others skilled in the art to which it appertains tomake and use the same.

This invention relates to a process of recovering a high gradephosphoric acid from the crude product usually obtained from treatingphosphate rock in fuel fedand electric furnaces, or from a combinedtreatment in fuel fed and electric furnaces, and

has for its object to provide a method of obtaining a high gradephosphoric acid which will be more efiicient and less costly than thoseheretofore proposed,

With these and. other objects view the invention consists in the novelsteps and combinations of steps constitutin Pmy in vention as will bemore fully; hereinafter disclosed and particularly pointed" out in theclaims.

In order that this invention may be more clearly understood it is said:In the manufacture of phosphoric acid in fuel fed or in electricfurnaces, there is ordinarily e1nployed a mixture of phosphate rock, orsinnlar phosphatic material and a silicious flux, with or without theaddition of carbonaceous material. This mixture is heated to a hightemperature, either by special means of combustion or by electriccurrent, and there is evolved more or less phosphorus pentoxid P 0 assuch, or in a reduced state, and in the form of a vapor which readilycondenses to a fume as the temperature of the gases carrying it drops.At the same time it is found impossible to prevent a portion of thecharge being volatilized, and another portion carried 0d as a fine dust,due to the rapid evolution of the gases from the body of the furnace,and to the high temperature therein. This volatilized foreign will befound to consist essentially of particles of phosphate rock,mechanically carried along with the gases, as well as volatilized limeCaO, silica SiO Specification of Letters Patent.

Patented Aug. 10, 1915.

Application filed June 9, 1914. Serial No. 844,018.

silicon fluorid, etc., but if reducing conditions are strong enough saiddust will contain some of the elements themselves. Air is alwaysintroduced into this dust either after it leaves the furnace, or beforeit leaves said furnace, and therefore these finely divided impuritiesare ultimately converted into oxids. Thesevolatilized materials are infact in just as fine a state of sub division as is the said phosphoruspentoxid,

0 so after condensation it is not possible to mechanically separate thevery finest dust and the volatilized material from the P 0 because ofthe fact, that they all have approximately the same specific gravity andare of the same magnitude in particle dimensions. They,

the electric furnace be used this said dust greatly increases in volumeon account of the higher temperature employed and therefore purificationin such case becomes of greater importance. It is true that a portion ofthe coarser particles of dust can be readily settled out, but suchportion constitutes a very small fraction of the whole amount ofmaterial carried along by the gas currents.

Should one employ an electric furnace,

the above phenomena will remain the same whether carbon is or is notused' in the charge, for somewhere in the operation an oxidizing actionmust be caused to take place in order to presentinto oxids before anabsorption of the phosphoric acid takes place. It therefore follows thatwhen this-said acid is formed it of necessity vmust contain lime,silico-phosphates, and other impurities which are soluble in the strongphosphoric acid that is present in large excess, and therefore itis'evident that a mere filtration of this said acid at this stage wouldonly partly purify it. In other Words, it is evident that thepurification of furnace made, or thermic, or electro-therinic madephosphoric acid is a very difierent problem from the purification ofphosphoric acid made. in the wet way as will further appear below.

In fact even if one operates his furnace under reducing conditions theend result will produceioxids such as P Q. CaO, SiO,, etc., all in avery finely divided condition convert all the vapors and which will givethe same troubles as those encountered under non reducing condltlons.

' will be foundeven in the case of alarge In fact, even if the vapors ofthe furnace containing these oxids are passed through absorption towersto recover the phosphoric acid in aqueous solution and to separate thelime, silica, and the dust" by. taking it up in the solution, one willstill obtain a very impure solution of phosphoric acid, for S idsolution will show considerable quantities of sludge containing lime,silica, phosphoric acid, and the other impurities. analysis of thissludge when filtered out will show that it some cases contains as. highas 20% of the total phosphoric acid .evolved from the furnace, and thishigh percentage unit thatis run at; the high tem eratures necessary toevolve P 0, wlth e c enc Inasmuch as it is sometimes desire to transformthis crude phosphoric acid over.

into salts of the alkali metals or of ammonia,

' .the presence -of these impurities becomes one or both of saidfurnaces.

' phosphoric acid.

particularly serious. is because upon neutralization with salts of thealkalis or of ammonia, there is formed a silico-phosphate, and dicalcium and tri-calcium phosphate, which carries down asins'olubleproducts a Very: large proportion of the phosphoric-acid made; and thewhole process thereupon become uncommercial for the production of alkalior ammonia salts of phosphoric acid.

In carrying out my invention I proceed as above disclosed, preferablyusing a fuel fed furnace for a primary smelting operation, and anelectric furnace as a finishing operation, and in order to insure theevolution of a very large percentage of the contained phosphorus, Ipreferably employ carbon in Further, in order to overcome the objectionsjust recited in connection withthe relatively large volume of finelydivided dust like impurities, I feed into the system sulfuric acid insuch quantities as to break up the silico-phosphates, thereby forming aelatinous. silica, and a phosphoric acid; whi e the lime phosphaticcompounds form calcium sulfate and Iv next separate these products fromthe free phosphoric acid either by settlement in the receivers in thebottom of the towers, or by filtration, inasmuch as it is a simplematter to filter phosphoric acid solutions because of theirnoncorrosive, properties when cold. This sulfuric acid addition tothecrudesolution can either be made directly to the absorbing watersused in the towers them selves, in which case it also assists in catch-7 ing a larger proportion of the P 0 than addition to the concentratedphosphoric acid a less dissolving power on calcium sulfate than theweaker acids obtained in the other towers, and, therefore, it ispossible to obtain a higher grade of phosphoric acid if the addition ismade to the acid after the concentration.

It will thus be seen that although the ordinary furnace processesheretofore proposed have roduced such large quantities of very finely'vided dust like material that it could -not be handled by any means ormethod which would produce commercial results yet by following theprocedure above disclosed I am enabled to get very high yields of posphorus from the original phosphate roc and to make theprocess commercially successful.

It is obvious that those skilled in the art may vary the details of myprocess without departing fromthe spirit thereof and therefore, I do notwish to be limited to the above disclosure except as may be required bythe claims.

What I claim is: v

1. The process of making a high grade phosphoric acid from phosphaterock, which consists in subjecting a suitable mixture containing saidrock and silica to the action of an electric furnace thereby producing afume containing oxids of phosphorus, calcium and silicon; subjectingsaid fume to the action of a sutficient quantity of sulfuric acid tobreak up any silico-phosphates that may be formed; and suitablyseparating out the free phosphoric acid thus produced, substantially asdescribed.

2. The process of making a high grade phosphoric acid'fromphosphaterock, which consists in subjecting a suitable mixture of said rock,silica and carbon, to the action of an electric furnace, therebyproducing a fume containing the oxids of phosphorus, calcium, andsilicon, as well as other impurities; subjecting said fume to the actionof water to form a' crude solution of hosphoric acid; adding to saidsolution su cient sulfuric acid to break up any silica-phosphates thatmay form: and suitably separating out the free phosphoric acid thusproduced, substantially as described.

3. The process of recovering a high grade phosphoric acid from crudephosphoric acid liquors containing the oxids of calcium and silicon,which consists in addin suificient sulfuric acid to said liquors whiithey are being formed to break up any silico-phosphates that may bepresent; and subsequently removing the insoluble impurities,substantially as described.

unease 4. The process of recoverin a high grade phosphoric acid fromcrude p osphoric acM liquors, containing compounds of calcium silicon;and other impurities, which consists in addin sufficient sulfuric acidto said liquors whi e the}? are being formed to cause insolubleprecipitates of the contained impurities and to break up anysilico-phosphat/es and phospiiatic compounds of calcium that would:otherwise exist; anti suitfie ably separating said insolubleprecipitates from said liquors; substantially as described. In testimonywhereof I afix my signature, in presence of two witnesses.

FRANK S. WASHBURN. Witnesses:

K. F. Gowns, S. WARREN MAY.

